Icom 751a: No Transmit

Discussion in 'Radio Circuits, Repair & Performance' started by 88MAN, Jul 24, 2017.

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  1. 88MAN

    88MAN QRZ Member

    My mint Icom 751a doesn't transmit -- No TX indicator light. No power. No carrier. Confirmed on meter and external MFJ 948 tuner/meter. I bought the rig new in 1991 and used it only for SWL for a few years. Life got in the way, and became too busy to get a license. So I stored the rig in an air tight bag for 26 yrs.

    I just received my Technician and General Licenses on the same day last week. Celebration has been short lived when I just tried to TX for the first time. I've replaced the memory board with the PIEXX module. After changing the board, the RIT displays 0, unless I press CL, then it works flawlessly. The RX works perfectly.

    I could really use some help as to what might the problem be? Any good Icom service techs.

    Thanks
     
  2. KQ4MM

    KQ4MM XML Subscriber QRZ Page

    Hi,

    I have a 751a thats actually being evaluated for possible repairs right now ... check this link for common symptoms for this unit

    http://www.qsl.net/icom/oldicomfaq/ic-751_51a.html

    Good luck, they typically suffer from dried out caps when they sit that long, also might want to look into a replacement memory board, the original battery is very difficult to replace and if if dies the unit is useless until a new programmed board is installed, there are at least 2 people who have the boards, just google 751a memory board.
     
  3. 88MAN

    88MAN QRZ Member

    Thank you KQ4MM! After 26 yrs, the memory board was still OK. However, I tried to transplant a new battery, but I melted the original board by soldering an umbiliical wire. I have consequently replaced the memory board with the PIEXX. Now the RIT displays 0 until I press CL. Then it works flawlessly. The rig doesn't have a single scratch on it anywhere. I found out that the rig is a high serial number and that the plastic caps might not be a problem. The electrolytics may be another issue. In any case, hopefully, I can find out what the problem(s) are, and have it restored.
     
  4. 88MAN

    88MAN QRZ Member

    Well, in any case, I've decided to ship it out to authorized Icom service in WA. The rig doesn't even have a single scratch on it. Besides, I don't feel like spending $2600 for a new rig with silly hidden menus. I looked at a Icom 9100 and was absolutely disgusted by the lack of quality - wobbly knobs and buttons, light weight plastics is a huge turn off. The SSB on the 751a sounds better. The 751a is build like a tank.
     
  5. VE3CGA

    VE3CGA XML Subscriber QRZ Page

     
  6. VE3CGA

    VE3CGA XML Subscriber QRZ Page

    I sent my 751A in to have the pll trimmer caps replaced and the other filter caps that are in the bulliten, I wanted it to be aligned at the factory and am glad I did, all wobbles, odd receive squiggles and sensitivity is better than I imagined. I could have replaced the caps myself but with no rf alignment equipment here I didnt want to 1/2 do it. its a great standby rig now and by the way I bought a new 9100 at the same time, its a great rig, and uses 1 usb cable for everything with the computer. all the sstv, wsjtx, fldigi, etc work very well. while the 751A sounds way better than it used to, this 9100 is like listening to hifi ham radio especially on 2M FM. Its on battery power until I finish building the supply but have been using FT8 on 6M and qrp'ing as well on 20m, never thought I would ever do qrp.
    the menus were a bit intimidating at first but after playing for a week, I have to say everything installed well, and although I still have to setup the wifi remote operating from my pool deck and the satelites to get into, the radio was a great retirement gift to myself. I dont think you will be disapppointed having Icom do the work but recommend having the pll trimmers changed and the rig aligned
    and I wish my IC9100 cost $2600 up here in Canada, I might have bought a 7300 with the difference.

    73's Bob VE3CGA
     
  7. 88MAN

    88MAN QRZ Member

    Thanks Bob, VE3CGA. I am encouraged to hear that the 9100 is a great performer, as you also own a 751a. It's really a shack in the box! It's versatility is unrivalled.
    I called Icom and they no longer service my radio. So they gave me the info for MTS in WA, who's an authorized Icom tech. He said since my serial # is high, 09761, that the trimmer caps around #10,000 seem to hold up better than early production ones, and they may be OK. Those PLL caps aren't easy to get to, requiring compete disassembly of the boards and shielding. I'm dumbfounded as to why Icom would cut corners on a $1 part?
    I just packaged the radio in its original Icom box. Ready to send it for restoration:
    1. Fix No Transmit issue - T9V regulator board.
    2. Change PLL caps with ceramic ones.
    3. Check output of finals, frequency stability, etc.
    4. Bring to factory spec.
     
  8. W9GB

    W9GB Ham Member QRZ Page

    Scott Malcom, owner/tech of MTS, worked for Icom Service when your IC-751A was new.
    He has some parts from that era, no longer available.
    http://mts-icom.wixsite.com/hamradiorepair

    The Icom IC-751A was one of Icom best HF transceivers, from that decade (1980s).
    These transceivers should never go to "chop shops" -- unless shipping damaged or water-logged.
     
  9. 88MAN

    88MAN QRZ Member

    Thanks W9GB! I could only look at a 751a on the Icom brochure way back when I was a student. I saved up for 2 years, then finally bought one in 1991, just before production ceased. I always felt it was an amazing radio of the highest standard of manufacturing. Same vintage as my Pioneer SX-3800 receiver, CT-950 tape deck, and Technics 1200 turntable. I can never part with this transceiver. I just can't seem to bring myself to buying a new rig with all kinds of hidden menus, and flimsy knobs, and plastic construction.

    As far as solder joints are concerned, well the new rigs use ROHS compliant lead-free solder because of the European regulations. This will make any newer radio less durable over the long haul than a pre-lead-free solder era like the 751a, 765, 781...

    P.S. I still have the 751a brochure. :)

    73, KC1HWT
     
  10. N8YX

    N8YX Ham Member QRZ Page

    I replied in the other active '751 thread...as a very satisfied owner/operator of this line of gear. Yes, it'll need a bit of attention (VCO trimmers, display circuitry, RAM board upgrades) but apart from these items it's a pretty solid design.

    Mine are keepers. So are a few other makes/models of that vintage which I own.

    Thanks to the "chop shops", I managed to get ahold of a few extra parts that may come in handy. Things like a display assembly (1ea for my '751A, R-71A and R-7000) and miscellaneous boards and controls.

    One of these days I ought to look at finding some NOS PA transistors for both the rig and amp but if one runs these conservatively there's no reason the original devices won't last another 40 years.
     

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